I have not been able to replicate on my end, so I would like to start by troubleshooting your browser. It is possible that this is being caused by something that has saved incorrectly in your browser history. I would suggest clearing your cache and cookies, then seeing if the issue continues.

I also noticed in the video you included that the preview of your mailing (in the upper left hand corner) is not loading. Does this happen frequently on your Review and Send page? This also indicates an issue, and may be resolved by clearing cache and cookies.